A surface acoustic wave convolver has been known as being a device utilizing nonlinearity of a surface acoustic wave medium wherein highly concentrated elastic energy can exist in a portion of the surface of the medium without spreading throughout the surface when a surface elastic wave travels along the surface. FIG. 1 is a theoretical diagram of a surface acoustic wave convolver wherein the reference numeral 1 designates a piezoelectric substrate, 2 and 3 denote a pair of input terminals provided at both sides of the substrate 1, and 4 denotes an output terminal disposed between the input terminals 2 and 3. Pulse signals applied to the input terminals 2 and 3, respectively, travel as surface acoustic waves along the surface of the piezoelectric substrate 1 toward the center and are taken up from the output terminal 4 as being convolution signals owing to nonlinearity of the substrate 1. In manufacturing such a surface acoustic wave convolver, it is preferable to enforce linearity of the piezoelectric substrate 1.
FIG. 2 shows a conventional convolver wherein the output terminal zone is constructed as being a nonlinear capacitance zone in order to emphasize the nonlinearity. In this Figure, the reference numeral 1 refers to a piezoelectric substrate, 5 to an input signal transducer including input signal terminals 5A and 5B, 6 to a reference signal transducer including reference signal terminals 6A and 6B, and 7 to a nonlinear capacitance zone, respectively. The nonlinear capacitance zone 7 includes a bias voltage terminal 8, convolution signal output terminals 9A and 9B, and plural pairs of bias resistors 10 and variable-capacitance diodes 11 which are connected in series between the bias voltage terminal 8 and the convolution signal output terminal 9A. This arrangement is advantageous in improvement of nonlinearity because it permits the nonlinear capacitance zone 7 to be designed independently from the travelling path of surface waves.
With the aforedescribed arrangement, however, improvement of convolution efficiency could not be effected easily because the variable-capacitance diodes 11 are two-terminal elements, so that it is difficult to control capacitance variation of the variable-capacitance diodes 11 themselves with respect to bias voltage as desired.